1. Field of the Invention
The present invention relates to a sensor element and a gas sensor.
2. Description of the Related Art
There is so far known a gas sensor for detecting the concentration of a specific gas, e.g., NOx, in measurement object gas to be measured, such as automobile exhaust gas. For example, Patent Literatures (PTL) 1 and 2 disclose a gas sensor including a sensor element that has an elongate plate-like shape, and that is formed by stacking a plurality of gas-tight oxygen ion-conductive solid electrolyte layers.
FIG. 15 is a schematic sectional view illustrating, in a simplified fashion, one example of a structure of a gas sensor 300 of the above-mentioned related art. As illustrated in FIG. 15, the gas sensor 300 includes a sensor element 307. The sensor element 307 is an element of a multilayer structure in which dense oxygen ion-conductive solid electrolyte layers 301 to 306 are stacked. In the sensor element 307, a measurement-object gas flowing portion through which measurement object gas is introduced is formed between a lower surface of the solid electrolyte layer 306 and an upper layer of the solid electrolyte layer 304. The measurement-object gas flowing portion includes a gas introducing region 310, and first to third inner cavities 320, 340 and 361. An inner pump electrode 322 is formed in the first inner cavity 320, an auxiliary pump electrode 351 is formed in the second inner cavity 340, and a measurement electrode 344 is formed on a lower surface of the third inner cavity 361 (i.e., an upper surface of the solid electrolyte layer 304). Furthermore, an outer pump electrode 323 is formed on an upper surface of the solid electrolyte layer 306. In the gas sensor 300, when the measurement object gas is introduced to the first inner cavity 320 in the measurement-object gas flowing portion, oxygen is pumped out or pumped in between the first inner cavity 320 and the outside of the sensor element 307 in accordance with a control voltage Vp0 that is applied between the outer pump electrode 323 and the inner pump electrode 322. Subsequently, when the measurement object gas is introduced to the second inner cavity 340, oxygen is pumped out or pumped in between the second inner cavity 340 and the outside of the sensor element 307 in accordance with a control voltage Vp1 that is applied between the outer pump electrode 323 and the auxiliary pump electrode 351. After the oxygen concentration of the measurement object gas has been controlled as described above during passage of the measurement object gas through the first inner cavity 320 and the second inner cavity 340, the measurement object gas is introduced to the third inner cavity 361. The concentration of a specific gas in the measurement object gas is then detected on the basis of a current Ip2 that flows when oxygen is pumped out or pumped in through the outer pump electrode 323 and the measurement electrode 344.
Moreover, the third inner cavity 361 is partitioned from the second inner cavity 340 by a partition wall 356. Diffusion rate-controlling portions 354 each having a slit-like shape are formed between the partition wall 356 and an upper surface of the measurement-object gas flowing portion (i.e., the lower surface of the solid electrolyte layer 306) and between the partition wall 356 and a lower surface of the measurement-object gas flowing portion (i.e., the upper surface of the solid electrolyte layer 304). The diffusion rate-controlling portions 354 give predetermined diffusion resistance to the measurement object gas that is introduced to the second inner cavity 340. Thus, an abrupt change in concentration of the measurement object gas reaching the measurement electrode 344 in the second inner cavity 340 is suppressed.